The following supplements the preliminary report in 11:04. Large steam plumes that sometimes contained a little ash have been observed at Shishaldin since mid-March. The most vigorous activity was reported on 7 May, when a gray plume with streaks of dark ash extended 130 km S of the volcano at 3.6 km altitude. Moderate steaming was continuing in early June.

On 19 March at about 1325, James Dickson saw a yellow-brown wispy plume rise about 90 m. A nearly transparent brownish layer that extended SW was visible for more than 50 km. MarkAir captain Jerry Chisum noted another ash layer from Shishaldin on 28 March. He reported that the volcano is usually steaming from the summit, but several times in the past several months he noticed anomalously large steam plumes with traces of ash.

Chisum and several other pilots observed larger-than-normal steam and ash emission from Shishaldin on 6 and 7 May. On both days (at 1320) the plume rose at about a 45° angle to 3,400 m altitude (550 m above the summit) then drifted horizontally for at least 25 km. On 6 May the plume emission was continuous, but on the 7th the plume was puffing roughly every 20 minutes. Reeve Aleutian Airways captain Lee Goch flew within 25 km of the volcano on 6 May at 1350, noting that the plume appeared to contain traces of ash as it was pale gray. His photograph showed that the plume had drifted at least 40 km NE at about 3,600 m altitude. On 7 May at about 1300, Aleutian Air pilot Thomas Madsen observed a plume extending at least 130 km S at about 3,600 m altitude. The plume was distinctly gray with some streaks of dark ash. Less intense activity was reported by both Madsen and Goch on 8 May, when Madsen saw a grayish-white plume that drifted E. On 10 May, Reeve Aleutian captain Edward Livingston observed only minor steaming. Similar activity, with none of the steam rising above the summit, was seen by Goch on 13 May. Two days later, he saw a 90-m plume that drifted ~4 km NE.

On 6 June at 1350, Livingston saw a white steam plume that rose 1100 m above the summit. The next day at the same time, the plume was 1400 m high; both drifted a few kilometers SE. On 9 June at 1340, the plume was only 180 m high but drifted 40 km E. Goch noted only minor summit steaming on 10 June.

Geological Summary. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: J. Reeder, ADGGS.